public class Solution {
    /*迭代*/
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        ListNode preHead = new ListNode(-1);
        ListNode curNode = preHead;
        while (l1 != null && l2 != null) {
            if (l1.val < l2.val) {
                curNode.next = l1;
                l1 = l1.next;
            } else {
                curNode.next = l2;
                l2 = l2.next;
            }
            curNode = curNode.next;
        }
        curNode.next = l1 == null ? l2 : l1;
        return preHead.next;
    }

    /*递归*/
    public ListNode mergeTwoLists2(ListNode l1, ListNode l2) {
        if (l1 == null) {
            return l2;
        } else if (l2 == null) {
            return l1;
        } else if (l1.val < l2.val) {
            l1.next = mergeTwoLists2(l1.next, l2);
            return l1;
        } else {
            l2.next = mergeTwoLists2(l1, l2.next);
            return l2;
        }

    }

}

class ListNode {
    int val;
    ListNode next;
    ListNode() {}
    ListNode(int val) { this.val = val; }
    ListNode(int val, ListNode next) { this.val = val; this.next = next; }
}